Auto Electronics

Freescale airbag sensor interface devices leverage high-speed DSI safety bus

At the Freescale Technology Forum in Tokyo this week Freescale Semiconductor ( introduced the MC33781 master interface and the MC33784 slave interface for remote sensing in airbag applications.

Automotive sensor operations manager Dave Monk said the devices target airbag safety systems that use the Distributed Systems Interface (DSI) automotive safety bus protocol to connect remotely placed pressure, acceleration, occupant, and buckle sensors to the main airbag electronic control unit (ECU).

Pioneered by Freescale and TRW (, the DSI protocol supports point-to-point, parallel, and daisy chain networks, and has been adopted by TRW, DENSO Corporation (, and other manufacturers of supplemental restraint system (SRS) airbags. Monk said Freescale’s new interface devices, which incorporate the next-generation DSI Version 2.02 protocol with differential drive (DBUS), were designed in collaboration with a key airbag system customer.

"We have used DSI-based devices for numerous airbag modules, and the demand for these devices will continue to increase to meet growing requirements for safety and advanced airbag systems,” said Mitsuhiko Masegi, managing officer, DENSO Corporation. “We believe Freescale’s new products for airbag applications will satisfy market needs, as well as future airbag requirements for high performance and functionality."

Manufactured on Freescale’s SMARTMOS 8 technology, the new DSI interface products offer more bus channels (four versus two) at a lower cost per-channel compared with earlier-generation devices. Prices were not disclosed. The new parts also feature an improved electromagnetic interference (EMI) signature, and higher top-end bus speeds (200 kbps versus 150 kbps).

The MC33781 master interface device provides the logic needed to interface the DSI buses to a standard serial peripheral interface (SPI) port – two SPI ports are available – and the analog circuitry to drive data and power over the bus as well as to receive data from remote slave devices like the MC33784.

The MC33781 features four independent differentialDSI bus channels, versus two, with a pseudo bus switch feature on channel 0. Monk said the devices’ differential operation is designed to reduce EMI and support fault tolerance and detection of shorts in the system. He added that independent frequency spreading on each channel further reduces interference by spreading the energy across many frequencies, thereby reducing the energy in any single frequency.
Fault detection features include overcurrent and overvoltage protection, and automatic message cyclical redundancy (CRC) generation and checking for each channel.

The DSI channels can support full duplex data transfers at up to 200 kbps versus 150 kbps. Monk said the faster speed could allow engineers to increase the number of sensors on a bus or to increase the sensor update rate. An addressable buffer enables queuing of four independent slave commands at a time for each channel.

MC33784 slave devices contain circuits to power a sensor, such as a two-axis accelerometer, and digitize the analog levels from the sensor. The devices are controlled by commands sent across the bus and they return measured data and other information over the same bus.

Each slave interface device contains two 10-bit analog inputs, three configurable logic inputs or outputs, and a regulated 5V output for powering remotely placed crash and occupant detection sensors.

Monk said the new interface devices address automakers’ need to enhance passenger safety while also reducing cost. He noted that airbag systems are becoming more sophisticated and, thus, requiring more sensors. The integration of pre-crash detection, electronic stability program (ESP) and other safety systems into airbag ECUs is also expected to increase sensor requirements.

Demand for airbag ECUs will reach 66 million by 2010, according to Strategy Analytics ( Airbag satellite demand will reach 132 million units; and the total available market (TAM) for silicon used in airbag systems will grow to $1.14 billion.

“With the move toward smaller, more fuel-efficient vehicles, the emphasis on safety features will continue to grow, and vehicle OEMs and their suppliers will increasingly adopt application-optimized standard protocols, such as DSI, to minimize the cost and complexity of implementing these enhanced systems,” said Chris Webber, Strategy Analytics vice president, global automotive practice. “We predict there will be some two billion nodes per year connected in car electronic networks, across all applications, by 2014.

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